HSV system (numeric values) broken?
No! There is not a single reason, why this should work. Manually adding HSV-System values will not necessary result in the same color as different other mixing methods.

Counterexample:
Mix the turquoise colors [H=0.49,S=1,V=1] and [H=0.51,S=1,V=1] and a similar turquoise color will be the result. But if you add the values the result is red [H=1,S=1,V=1]

The reason for this behavior is purely based on the mathematics of color mixing and is not a question concerning blender.